Thin laminate flooring and wood veneer flooring are usually composed of a body consisting of a 6-9 mm fibreboard, a 0.2-0.8-mm-thick upper surface layer and a 0.1-0.6 mm lower balancing layer. The surface layer provides appearance and durability to the floorboards. The body provides stability, and the balancing layer keeps the board level when the relative humidity (RH) varies during the year. The RH can vary between 15% and 90%. Conventional floorboards of this type are usually joined by means of glued tongue-and-groove joints at the long and short sides. When laying the floor, the boards are brought together horizontally, whereby a projecting tongue along the joint edge of a first board is introduced into the tongue groove along the joint edge of a second board. The same method is used on both the long and the short side. The tongue and the tongue groove are designed for such horizontal joining only and with special regard to how the glue pockets and gluing surfaces should be designed to enable the tongue to be efficiently glued within the tongue groove. The tongue-and-groove joint presents coacting upper and lower contact surfaces that position the boards vertically in order to ensure a level surface of the finished floor.
In addition to such conventional floors which are connected by means of glued tongue-and-groove joints, floorboards have recently been developed which are instead mechanically joined and which do not require the use of glue. This type of a mechanical joint system is hereinafter referred to as a “strip-lock system” since the most characteristic component of this system is a projecting strip which supports a locking element.
WO 94/26999 (Applicant Valinge Aluminum AB) discloses a strip-lock system for joining building panels, particularly floorboards. This locking system allows the boards to be locked mechanically at right angles to as well parallel to the principal plane of the boards at the long side as well as at the short side. Methods for making such floorboards are disclosed in WO 98/24994 and WO 98/24995. The basic principles of the design and the installation of the floorboards, as well as the methods for making the same, as described in the three above-mentioned documents are usable for the present invention as well, and, therefore, these documents are hereby incorporated by reference.
In order to facilitate the understanding and description of the present invention, as well as the comprehension of the problems underlying the invention, a brief description of the basic design and function of the floorboards according to the above-mentioned WO 9426999 will be given below with reference to FIGS. 1-3 in the accompanying drawings. Where applicable, the following description of the prior art also applies to the embodiments of the present invention described below.
FIGS. 3a and 3b are thus a bottom view and a top view respectively of a known floorboard 1. The board 1 is rectangular with a top side 2, an underside 3, two opposite long sides 4a, 4b forming joint edges, and two opposite short sides 5a, 5b forming joint edges.
Without the use of glue, both the long sides 4a, 4b and the short sides 5a, 5b can be joined mechanically in a direction D2 in FIG. 1c. For this purpose, the board 1 has a flat strip 6, mounted at the factory, projecting horizontally from its long side 4a, which strip extends throughout the length of the long side 4a and which is made of flexible, resilient sheet aluminum. The strip 6 can be fixed mechanically according to the embodiment shown, or by means of glue, or in some other way. Other strip materials can be used, such as sheets of other metals, as well as aluminum or plastic sections. Alternatively, the strip 6 may be made in one piece with the board 1, for example by suitable working of the body of the board 1. Thus, the present invention is usable for floorboards in which the strip is integrally formed with the board. At any rate, the strip 6 should always be integrated with the board 1, i.e. it should never be mounted on the board 1 in connection with the laying of the floor. The strip 6 can have a width of about 30 mm and a thickness of about 0.5 mm. A similar, but shorter strip 6′ is provided along one short side 5a of the board 1. The edge side of the strip 4 facing away from the joint edge 4a is formed with a locking element 8 extending throughout the length of the strip 6. The locking element 8 has an operative locking surface 10 facing the joint edge 4a and having a height of e.g. 0.5 mm. When the floor is being laid, this locking surface 10 coacts with a locking groove 14 formed in the underside 3 of the opposite long side 4b of an adjoining board 1′. The short side strip 6′ is provided with a corresponding locking element 8′, and the opposite short side 5b has a corresponding locking groove 14′.
Moreover, for mechanical joining of both the long sides and the short sides also in the vertical direction (direction D1 in FIG. 1c), the board 1 is formed with a laterally open recess 16 along one long side 4a and one short side 5a. At the bottom, the recess is defined by the respective strips 6, 6′. At the opposite edges 4b and 5b, there is an upper recess 18 defining a locking tongue 20 coacting with the recess 16 (see FIG. 2a).
FIGS. 1a-1c show how two long sides 4a, 4b of two such boards 1, 1′ on an underlay U can be joined together by means of downward angling. FIGS. 2a-2c show how the short sides 5a, 5b of the boards 1, 1′ can be joined together by snap action. The long sides 4a, 4b can be joined together by means of both methods, while the short sides 5a, 5b—when the first row has been laid—are normally joined together subsequent to joining together the long sides 4a, 4b and by means of snap action only.
When a new board 1′ and a previously installed board 1 are to be joined together along their long sides 4a, 4b as shown in FIGS. 1a-1c, the long side 4b of the new board 1′ is pressed against the long side 4a of the previous board 1 as shown in FIG. 1a, so that the locking tongue 20 is introduced into the recess 16. The board 1′ is then angled downwards towards the subfloor 12 as shown in FIG. 1b. In this connection, the locking tongue 20 enters the recess 16 completely, while the locking element 8 of the strip 6 enters the locking groove 14. During this downward angling the upper part 9 of the locking member 8 can be operative and provide guiding of the new board 1′ towards the previously installed board 1. In the joined position as shown in FIG. 1c, the boards 1, 1′ are locked in both the direction D1 and the direction D2 along their long sides 4a, 4b, but can be mutually displaced in the longitudinal direction of the joint along the long sides 4a, 4b. 
FIGS. 2a-2c show how the short sides 5a and 5b of the boards 1, 1′ can be mechanically joined in the direction D1 as well as the direction D2 by moving the new board 1′ towards the previously installed board 1 essentially horizontally. Specifically, this can be carried out subsequent to joining the long side of the new board 1′ to a previously installed board in an adjoining row by means of the method according to FIGS. 1a-1c. In the first step in FIG. 2a, beveled surfaces adjacent to the recess 16 and the locking tongue 20 respectively co-operate such that the strip 6′ is forced to move downwards as a direct result of the bringing together of the short sides 5a, 5b. During the final urging together of the short sides, the strip 6′ snaps up when the locking element 8′ enters the locking groove 14′.
By repeating the steps shown in FIGS. 1a-c and 2a-c, the whole floor can be laid without the use of glue and along all joint edges. Known floorboards of the above-mentioned type are thus mechanically joined usually by first angling them downwards on the long side, and when the long side has been secured, snapping the short sides together by means of horizontal displacement along the long side. The boards 1, 1′ can be taken up in the reverse order of laying without causing any damage to the joint, and be laid again. These laying principles are also applicable to the present invention.
For optimal function, subsequent to being joined together, the boards should be capable of assuming a position along their long sides in which a small play can exist between the locking surface 10 and the locking groove 14. Reference is made to WO 9426999 for a more detailed description of this play.
In addition to what is known from the above-mentioned patent specifications, a licensee of Valinge Aluminum AB, Norske Skog Flooring AS (NSF), introduced a laminated floor with mechanical joining according to WO 9426999 in January 1996 in connection with the Domotex trade fair in Hannover, Germany. This laminated floor, which is marketed under the brand name Alloc™, is 7.2 mm thick and has a 0.6-mm aluminum strip 6 which is mechanically attached on the tongue side. The operative locking surface 10 of the locking element 8 has an inclination (hereinafter termed locking angle) of 80° to the plane of the board. The vertical connection is designed as a modified tongue-and-groove joint, the term “modified” referring to the possibility of bringing the tongue and tongue groove together by way of angling.
WO 97/47834 (Applicant Unilin) describes a strip-lock system which has a fibreboard strip and is essentially based on the above known principles. In the corresponding product, “Uniclic”, which this applicant began marketing in the latter part of 1997, one seeks to achieve biasing of the boards. This results in high friction and makes it difficult to angle the boards together and to displace them. The document shows several embodiments of the locking system. The “Uniclic” product, shown in section in FIG. 4b, consists of a floorboard having a thickness of 8.1 mm with a strip having a width of 5.8 mm, comprising an upper part made of fibreboard and a lower part composed of the balancing layer of the floorboard. The strip has a locking element 0.7 mm in height with a locking angle of 45°. The vertical connection consists of a tongue and a tongue groove having a tongue groove depth of 4.2 mm.
Other known locking systems for mechanical joining of board materials are described in, for example, GB-A-2,256,023 showing unilateral mechanical joining for providing an expansion joint in a wood panel for outdoor use, and in U.S. Pat. No. 4,426,820 showing a mechanical locking system for plastic sports floors, which floor however does not permit displacement and locking of the short sides by snap action. In both these known locking systems the boards are uniform and do not have a separate surface layer and balancing layer.
In the autumn of 1998, NSF introduced a 7.2-mm laminated floor with a strip-lock system which comprises a fibreboard strip and is manufactured in accordance with WO 9426999. This laminated floor, which is shown in cross-section in FIG. 4a, is marketed under the brand name of “Fiboloc™”. In this case, too, the strip comprises an upper part of fibreboard and a lower part composed of a balancing layer. The strip is 10.0 mm wide, the height of the locking element is 1.3 mm and the locking angle is 60°. The depth of the tongue groove is 3.0 mm.
In January 1999, Kronotex introduced a 7.8 mm thick laminated floor with a strip lock under the brand name “Isilock”. This system is shown in cross-section in FIG. 4c. In this floor, too, the strip is composed of fibreboard and a balancing layer. The strip is 4.0 mm and the tongue groove depth is 3.6 mm. “Isilock” has two locking ridges having a height of 0.3 mm and with locking angles of 40°. The locking system has low tensile strength, and the floor is difficult to install.